I. Field of the Invention
The present invention is directed to piston engines, and more particularly, to aircraft engines having a rust-resistant cylinder wall with superior piston ring seating properties.
II. Description of the Prior Art
Piston engines, and in particular internal combustion engines, typically comprise a multiplicity of cylinder assemblies. These cylinder assemblies conventionally comprise at least one cylinder head and at least one steel cylinder attached to the head. Because the weight of the engine is a significant factor for the overall performance and efficiency of the engine, attempts are often made to reduce the overall weight of the engine. In aircraft engines, for example, the cylinder head or heads are often constructed of aluminum, or an aluminum or other alloy. For various reasons, steel remains the material of choice for the cylinder itself.
One problem with the steel cylinders conventionally used in aircraft engines is that the cylinders are susceptible to rust or rust-type corrosion of the cylinder bores, especially during storage or during the first few hours of operation of the engine. The problem is particularly acute when the engine is run briefly, for example, after assembly, and then allowed to sit idle for some period of time. If this corrosion is allowed to develop, pits may be formed in the cylinder barrel surface. This corrosion is sometimes severe enough to adversely effect piston ring wear and oil control.
There are, of course, several ways of inhibiting rust formation on a steel surface, but the nature of the cylinder assembly or aircraft engines renders these ways less than useful. Painting the cylinder walls may interfere with the sealing function of the piston rings, and can be abraded from the wall during the first few hours of operation. Provision of an oxide coating on the cylinder barrels may not be possible, because of the nature of typical aircraft engine manufacturing procedures. One of the first steps assembling an engine is to assemble the steel cylinder or cylinders to the aluminum cylinder head. The chemicals employed in creating an oxide layer on the barrel surface are often corrosive to aluminum, and their use is therefore prohibited.
The application of a layer of crystalline phosphate to a ferrous surface is known, for example, to a steel engine combustion chamber head, as in U.S. Pat. No. 3,082,128 to Craig. Craig disclosed phosphatizing the combustion chamber head for the purpose of reducing carbon buildup in the head during operation of the engine. But no reference has been found in which the cylinder barrel, and in particular the cylinder wall, has been phosphatized.
Moreover, as noted by Blum et al in U.S. Pat. No. 3,297,493, at column 1, lines 28 through 33, phosphate-type coatings are usually characterized as being relatively fragile. A phosphorous coating on a cylinder wall could be expected to wear away during operation of the engine, under the abrasive friction of the piston rings, or could be expected to adversely effect piston ring wear. The potential for interference with the sealing function of the piston rings could also be expected from such a coating.